Question

what is meant by valency of an element?what is the relationship between the number of valence electron and valency

Answer 1

Ans: The combining capacity of an atom is called its valency. Actually it can be defined as the number of electrons that an atom may lose (or) gain during a chemical reaction (or) the number of electrons shared.
Ex: In water molecule H₂O (H - O – H)
  Valency of Hydrogen is ‘ 1 ‘ (as it combines with Oxygen atom by  one single bond) and valence electrons in Hydrogen  = 1
 Valency of Oxygen is ‘2’ (as it combines with two Hydrogen atoms by two bonds), but valence electrons in oxygen  = 6
The numbers of electrons in outermost shell (or) valency shell of an atom are called valency electrons.
Based on valency electrons the group number of an atom can be found.
Ex: Number of valence electrons in chlorine are 7 so it belongs to VIIA - group, but its valency is 1.
 
So, it is to be understood that some times valency is equal to valence electrons, but not all the time.

Answer 2

The number of electrons in an atom is always the same as the number of protons, so atoms are electrically neutral over all. Atoms can loss or gain electrons. If an atom gains one or more electrons, it becomes a negativity changed ions